a. Field of the Invention
This invention relates to a push-pull type transistor amplifier circuit using complementary transistors and more particularly to an improvement of the signal-to-noise (S/N) ratio thereof.
B. Description of the Prior Art
FIG. 1 shows an example of a complementary single-ended push-pull amplifier circuit disclosed in the Japanese Pat. Application No. 50-104085. In this amplifier circuit, collectors of complementary transistors Q1 and Q2 are directly connected with each other; emitters thereof are respectively connected with positive and negative supply voltage sources +E and -E through resistors R1 and R2; and bases thereof are connected with each other and with the common collector through a feed-back resistor R3. Capacitors C3 and C4 form by-pass circuits and capacitors C1 and C2 are coupling capacitors for an input terminal IN and for an output terminal OUT. In such a circuit, collectors of transistors Q1 and Q2 are kept at the mid-potential, i.e. at ground potential, and little current is allowed to flow through the resistor R3 when there is no signal and the potential difference between the collectors and the bases of the transistors Q1 and Q2 is almost zero. Therefore, the leakage current I.sub.CBO between the collector and the base of the transistors Q1 and Q2 is nearly zero so that the transistors Q1 and Q2 scarecely produce excess noises and hence the S/N ratio is remarkably improved.
The previously disclosed circuit as mentioned above has an advantage of effectively suppressing the generation of excess noises in the transistors Q1 and Q2. On the other hand, however, it is necessary to connect resistors R1 and R2 with the emitters of the transistors Q1 and Q2 for determining the collector current of the transistors Q1 and Q2 and compensating for the temperature dependence thereof. The thermal noise produced in the resistors R1 and R2 may form a factor for the deterioration of the S/N ratio, and the utilization, i.e. the effective range, of the supply voltage tends to decrease due to the voltage drop in the resistors R1 and R2. Besides, by-pass capacitors C3 and C4 become necessary for the insertion of resistors R1 and R2 and generally should have large capacitances since the impedance is fairly low. Furthermore, when the characteristics of the transistors Q1 and Q2 differ from each other, the two resistors R1 and R2 must be variable resistors, which makes the circuit construction more complicated.